1. Field of the Invention
This invention relates generally to the field of metal-to-metal antifuses which are used in integrated circuit devices to form selectively programmable conductive links between metallization layers of the integrated circuit devices. More particularly, this invention relates to improved antifuses and methods for programming such antifuses which yield, after programming, an antifuse link disposed through the antifuse material layer which does not include any significant quantities of aluminum metal and which is, therefore, more reliable and of more predictable resistance.
2. The Prior Art
Antifuses and metal-to-metal antifuses are well known in the art. A problem exists, however, which affects the useful life of devices incorporating metal-to-metal antifuses. Most metal-to-metal antifuses exist between two aluminum metal film metallization layers in an integrated circuit device such as a CMOS-type device. When a voltage of sufficient magnitude is applied between the metallization layers in the vicinity of the antifuse, the antifuse material layer and a portion of the adjacent metallization layers will disrupt and/or melt or vaporize and a conductive link will form through the antifuse material layer due to metal from the adjacent metallization layer being drawn in and intermixed through mass transport and thermal driven material diffusion and chemical reaction, effectively shorting the two metallization layers together at the location of the antifuse. The conductive link is primarily formed of the material in the adjacent metallization layer in the immediate vicinity of the disruption.
Unfortunately, in many cases it has been noticed that under certain types of stress such conductive links will open up or become non-conductive. This failure mode often manifests itself after a period of time during which the antifuse links appear to have been properly formed. This failure mode is commonly known as "read disturb" because the small currents used to read the state of the antifuse--much smaller than the typical current used to program the antifuse--eventually cause a state reversal in the programmed antifuse. Such a failure can destroy not only the particular link that fails, but the entire device in which the antifuse is located and, potentially, the equipment in which the device is placed. This problem has substantially retarded the commercial acceptance and development of metal-to-metal antifuses.
One factor that is now believed to contribute to read disturb is the presence of any significant quantity of aluminum in the antifuse conductive links. Aluminum is well known to be subject to electromigration, i.e., in the presence of an electron current flow, the aluminum metal atoms tend to move along with the electron current flow (opposite to the direction of normal positive to ground electric current flow). Accordingly, researchers in the prior art have attempted to block aluminum flow into the antifuse material layer with barrier layers of various materials and various thicknesses. The barrier metals, in turn, have to take the place of the aluminum, and provide essentially the entirety of the conductive material which flows into the antifuse material layer to form the conductive links that short the two metallization layers together.
During the programming of the antifuse, the resistance between the two metallization layers of the antifuse goes from a few gigaohms to less than 100 ohms--a change of over seven orders of magnitude. A relatively large amount of heat is also released during this process. This heat, if near enough to the aluminum metallization layer, can melt or vaporize it, making it easy for aluminum to flow into the antifuse material layer to undesirably participate in the formation of conductive links.
One prior art method for forming antifuses is illustrated in FIG. 1. According to this method, an antifuse aperture or via 10 is formed between two metallization layers 12, 14 separated by a dielectric material 16 such as SiO.sub.2 ("interlayer dielectric layer"). In this type of structure, the lower antifuse electrode 18 is formed of an aluminum metallization layer 12 covered by a barrier metal layer 20; the upper antifuse electrode 22 is formed of aluminum metallization layer 14 disposed over barrier metal layer 24. Antifuse material layer 26 is deposited into the antifuse aperture 10 with any of a number of techniques and may comprise any of a number of materials including multiple layers of different materials, and is subject to step coverage problems which occur whenever a material is deposited into an aperture, namely, a thinning of the deposited layers at the edges or corners of the aperture 28, 30 relative to the thickness of the layer at the center 32 of the aperture 10. This thinning of both antifuse material layer 26 and barrier metal layer 24 tends to force the antifuse to "blow" or "program" in one of the corners 28, 30 at a somewhat unpredictable voltage and can lead to aluminum from metallization layer 14 breaching the thinned barrier metal layer 24 to enter antifuse material layer 26 during the programming of the antifuse and thereby contaminate the conductive link with aluminum metal.
A similar antifuse structure, known as the "half-stack", is shown in FIG. 2. The FIG. 2 antifuse 34 comprises a lower electrode 36 formed of an aluminum metallization layer 38 overlaid by a layer of a barrier metal 40. Over the barrier metal layer 40 is disposed an antifuse material layer 42 which may be of a multi-layer multi-material construction or other construction as is well known in the art. Over the antifuse material layer 42 is an antifuse via 44 through interlayer dielectric layer 52 into which is deposited upper electrode 46 which consists of barrier metal layer 48 and aluminum metallization layer 50. An interlayer dielectric layer 52 is disposed between the electrodes 36, 46. Such an antifuse may be programmed as follows: provide a first voltage pulse across electrodes 36, 46 with the less positive voltage tied to electrode 46 and the more positive voltage tied to electrode 36, the difference between the less positive and more positive voltages being a potential sufficient to program the antifuse, followed by a series of other voltage pulses, known as an "ac soak" (See, e.g., U.S. patent application Ser. No. 08/110,681 filed Aug. 23, 1993 in the name of Steve S. Chiang, et al., entitled "Methods For Programming Antifuses Having at Least One Metal Electrode", which is hereby incorporated herein by reference), to aid in the formation of the conductive link. This approach of applying the more positive (greater) voltage first to the bottom electrode is known as "VOB" or voltage on bottom. The converse arrangement is known as "VOT" or voltage on top. A method often used in programming antifuses is to apply a series of programming pulses having an amplitude in one direction greater than the amplitude in the other direction, e.g., +10 volt pulse followed by -8 volt pulse. In this way, the greater stress is applied during the +10 volt pulse, thus the antifuse is more likely to program during the +10 volt pulse. If it fails to program on the first pulse, it probably won't program on the -8 volt pulse and will likely program on a subsequent +10 volt pulse. In this way it can be assured that the antifuse will only disrupt while the current is flowing in one particular and predetermined direction.
As used herein, the terms VOT and VOB refer to either a pure DC programming voltage, or more commonly, to the first pulse, or to the pulse having the greater magnitude when it is not the first pulse. Thus VOT and VOB refer to the conditions during actual initial formation of the conductive link.
When a series of half-stack antifuses having upper barrier layers of TiN of minimum thickness of less than about 2000 .ANG. were programmed in this fashion, VOB, the results shown in FIG. 3 were obtained FIG. 3 is a histogram plot showing the resulting resistance of the programmed antifuse along the horizontal axis against the number of antifuses having that resistance along the vertical axis. As can be seen, the grouping is not very tight with an average of 21 ohms, an outlier at nearly 100 ohms and a standard deviation of 3.1. Aluminum content in the conductive links is believed responsible for this relatively poor performance.
FIG.9A and FIG. 9B are renditions of scanning electron micrographs of sectioned, programmed antifuses. In FIGS. 9A and FIGS. 9B, antifuses comprising upper aluminum metal layer A, barrier layer B, antifuse layer C, lower barrier layer D and lower aluminum metal layer E are shown. The antifuse of FIG. 9A was programmed VOT resulting in visible disruption/mixing of layers D and C but no visible effects to layer B. In this case, a more positive voltage applied to the top electrode (A/B) produces a downward electric current and an upward electron current. The result is a conductive filament shorting layers B and D. If the disruption (shown at "X") of layer D is sufficient to extend all of the way through layer D (which it can be seen that it does not do here), aluminum in layer E will also likely be disrupted/intermixed and the potential for inclusion of aluminum in the conductive filament is substantially increased. In FIG. 9B the antifuse was programmed VOB and the opposite results obtain. Layer D is apparently entirely unscathed, but layers B and C are disrupted. Because such via-type antifuses have well known step coverage problems, the thickness of layer B is thinner in the corners of the antifuse aperture, hence the antifuse tends to program in the corners due to less overall electrical resistance in these locations. As can be seen, layer B is significantly eroded to the point of almost complete penetration where the antifuse programmed.
U.S. Pat. No. 5,302,546 to Gordon, et al., describes a possible solution to this problem. In FIG. 1 of Gordon, et al., reproduced here as FIG. 4, an antifuse 54 is shown which incorporates non-conductive spacers 56, 58 disposed in the corners 60, 62, respectively, of the antifuse aperture 64. These spacers 56, 58 force the conductive metal electrode 66 formed of barrier metal 68 and aluminum metallization layer 70 away from the corners 60, 62 so that it is disposed immediately above only the thickest part 72 of the antifuse material layer 74. Antifuse 54 is constructed over a lower electrode 78 which is disposed over dielectric layer 80 which is in turn disposed over substrate 82. Interlayer dielectric layer 84 separates electrodes 66 and 78. An important drawback to this solution is that formation of spacers 56, 58 requires additional process steps which increase costs. Gordon, et al. further teach using a VOT programming scheme with their antifuse, however, they do not suggest why this is to be preferred to a VOB scheme, nor do they teach any utility in providing differential barrier metal layer thicknesses in conjunction with such a scheme or a realization that the spacers 56, 58 may be obviated by choice of materials and programming technique.
Accordingly, it would be extremely desirable to formulate a design for a metal-to-metal via-type antifuse which is not susceptible to conductive link failure or "read-disturb" as a solution to this problem would open the door to wide commercial acceptance of metal-to-metal antifuses.